Circuit for performing external pacing and biphasic defibrillation

ABSTRACT

An external defibrillator/pacer ( 8 ) includes an output circuit ( 14 ) with four legs arrayed to form an H-bridge. Each leg of the output circuit contains a switch (SW 1 -SW 4 ). In a defibrillation mode, pairs of switches in the H-bridge are selectively switched to generate a biphasic defibrillation pulse. Three switches (SW 1 , SW 3 , SW 4 ) are silicon controlled rectifiers (SCRs). Gate drive circuits ( 51, 53, 54 ) are coupled to the SCRs to bias the SCRs with a voltage that allows the SCRs in response to control signals. One switch (SW 2 ) includes an insulated gate bipolar transistor (IGBT). A gate drive circuit ( 52 ) is coupled to the gate of the IGBTs to provide a slow turn-on and a fast turn-off of the IGBT. In a pacing mode, a bypass circuit or current source circuit is used to provide a current path bypassing an SCR switch (SW 3 ), which cannot be triggered by the relatively low current of pacing pulses. One of the SCRs (SW 4 ) may be replaced with an IGBT to allow generation of the pacing pulse with opposite polarity of the first phase of the defibrillation pulse.

FIELD OF THE INVENTION

[0001] This invention relates generally to apparatus for generatingstimulation waveforms, and more particularly to a circuit for generatingboth pacing and defibrillation waveforms in an external unit.

BACKGROUND

[0002] One of the most common and life-threatening medical conditions isventricular fibrillation, a condition where the human heart is unable topump the volume of blood required by the human body. The generallyaccepted technique for restoring a normal rhythm to a heart experiencingventricular fibrillation is to apply a strong electric pulse to theheart using an external cardiac defibrillator. External cardiacdefibrillators have been successfully used for many years in hospitalsby doctors and nurses, and in the field by emergency treatmentpersonnel, e.g., paramedics.

[0003] Conventional external cardiac defibrillators first accumulate ahigh-energy electric charge on an energy storage capacitor. When aswitching mechanism is closed, the stored energy is transferred to apatient in the form of a large current pulse. The current pulse isapplied to the patient via a pair of electrodes positioned on thepatient's chest. The switching mechanism used in most contemporaryexternal defibrillators is a high-energy transfer relay. A dischargecontrol signal causes the relay to complete an electrical circuitbetween the storage capacitor and a wave shaping circuit whose output isconnected to the electrodes attached to the patient.

[0004] The relay used in contemporary external defibrillators hastraditionally allowed a monophasic waveform to be applied to thepatient. It has recently been discovered, however, that there may becertain advantages to applying a biphasic rather than a monophasicwaveform to the patient. For example, preliminary research indicatesthat a biphasic waveform may limit the resulting heart trauma associatedwith the defibrillation pulse.

[0005] The American Heart Association has recommended a range of energylevels for the first three defibrillation pulses applied by an externaldefibrillator. The recommended energy levels are: 200 joules for a firstdefibrillation pulse; 200 or 300 joules for a second defibrillationpulse; and 360 joules for a third defibrillation pulse, all within arecommended variance range of no more than plus or minus 15 percentaccording to standards promulgated by the Association for theAdvancement of Medical Instrumentation (AAMI). These high energydefibrillation pulses are required to ensure that a sufficient amount ofthe defibrillation pulse energy reaches the heart of the patient and isnot dissipated in the chest wall of the patient.

[0006] On the other hand, pacers are typically used to administer aseries of relatively small electrical pulses to a patient experiencingan irregular heart rhythm. For example, each pacing pulse typically hasan energy of about 0.05 J to 1.2 J. Because of the small energies usedfor pacing pulses, the circuitry used to generate the pacing pulsescannot typically be used for generating defibrillation pulses.

[0007] There are some systems that combine both a pacer and adefibrillator in a single unit for providing pacing pulses anddefibrillation pulses as required. These conventional systems typicallyuse separate defibrillation and pacing generation circuits. For example,FIG. 1 shows a combined pacing defibrillation unit 5 having adefibrillation circuit 6 and a pacing circuit 7. Unit 5 selectivelydelivers defibrillation or pacing pulses to the patient. Implantablesystems generally use separate electrodes for pacing and defibrillation.An example of an implantable combined defibrillator/pacer is found inU.S. Pat. No. 5,048,521. Of course, having separate defibrillation andpacing circuits tends to increase the cost and size of the unit. Inaddition, because implantable defibrillators and pacers typically applyrelatively low energy pulses, the output circuitry for such implantableunits is generally not adaptable for use in an external unit.

[0008] The present invention is directed to an apparatus that overcomesthe foregoing and other disadvantages in an externalpacing/defibrillation unit. More specifically, the present invention isdirected to a single output circuit for an external pacer/defibrillatorthat is capable of applying both high-energy biphasic defibrillationpulses and low-energy pacing pulses to a patient.

SUMMARY

[0009] In accordance with the present invention, an externaldefibrillator/pacer having an output circuit that is used in generatingboth a defibrillation pulse and a pacing pulse is provided. The outputcircuit includes four legs arrayed in the form of an “H” (hereinafterthe “H-bridge output circuit”). Each leg of the output circuit containsa solid-state switch. By selectively switching on pairs of switches inthe H-bridge output circuit, biphasic or monophasic defibrillation andpacing pulses may be applied to a patient.

[0010] In accordance with one aspect of the invention, the switches inthree of the legs of the H-bridge output circuit are silicon controlledrectifiers (SCRs). A single SCR is used in each of these three legs. Theswitch in the fourth leg is an insulated gate bipolar transistor (IGBT).The bypass circuit is capable of conducting the relatively small pacingcurrents, which are generally too small to trigger the SCRs required toconduct the relatively large defibrillation currents. The addition ofthe bypass circuit eliminates the need for separate defibrillation andpacing output circuits.

[0011] In accordance with another aspect of the invention, the H-bridgeoutput circuit has two IGBT legs and two SCR legs. The second IGBT legallows the polarity of the defibrillation and pacing pulses to beopposite. In one embodiment, the pacing current is adjusted by adjustingthe voltage on an energy storage capacitor.

[0012] In accordance with yet another aspect of the invention, insteadof a bypass circuit, an adjustable current source is used to provide thepacing current. This current source is coupled to the energy storagecapacitor. In one embodiment, the current source is an IGBT operated inthe linear region.

[0013] In accordance with still another aspect of the invention, all ofthe H-bridge legs are implemented with IGBTs. This aspect allows forgeneration of biphasic pacing pulses. Further, by biasing the IGBTs inthe linear region, the IGBTs can be used as current sources to controlthe pacing current. This would eliminate the need for a bypass circuitor separate current source.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The foregoing aspects and many of the attendant advantages ofthis invention will become more readily appreciated by reference to thefollowing detailed description, when taken in conjunction with theaccompanying drawings listed below.

[0015]FIG. 1 is a block diagram of a conventional combineddefibrillator/pacer unit.

[0016]FIG. 2 is a block diagram illustrative of a combineddefibrillator/pacer unit having a single output circuit, according toone embodiment of the present invention.

[0017]FIG. 3 is a flow diagram illustrative of the operation of thecombined defibrillator/pacer unit depicted in FIG. 2.

[0018]FIG. 4 is a more detailed block diagram illustrative of thecombined defibrillator/pacer unit depicted in FIG. 2.

[0019]FIG. 5 is a schematic diagram illustrative of the block diagramdepicted in FIG. 4.

[0020]FIG. 6 is a flow diagram illustrative of the operation of thecombined defibrillator/pacer unit depicted in FIG. 5.

[0021]FIG. 7 is a diagram illustrative of the waveforms generated by thecombined defibrillator/pacer unit depicted in FIG. 5, according to oneembodiment of the present invention.

[0022]FIG. 8 is a block diagram illustrative of another embodiment of acombined defibrillator/pacer unit, according to the present invention.

[0023]FIG. 9 is a diagram illustrative of the waveforms generated by thecombined defibrillator/pacer unit depicted in FIG. 8, according to oneembodiment of the present invention.

[0024]FIG. 10 is a schematic diagram of an IGBT driver for linearcontrol of an IGBT, according to one embodiment of the presentinvention.

[0025]FIG. 11 is a flow diagram illustrative of the operation of thecombined defibrillator/pacer unit depicted in FIG. 8.

[0026]FIG. 12 is a block diagram illustrative of a combineddefibrillator/pacer unit, according to yet another embodiment of thepresent invention.

[0027]FIG. 13 is a diagram illustrative of various waveforms generatedby the combined defibrilator/pacer unit depicted in FIG. 12.

[0028]FIG. 14 is a block diagram illustrative of a combineddefibrillator/pacer unit, according to still another embodiment of thepresent invention.

[0029]FIG. 15 is a diagram illustrative of current sensing circuit,according to one embodiment of the present invention.

DETAILED DESCRIPTION

[0030]FIG. 2 is a block diagram illustrative of an external combineddefibrillator/pacer 8, according to one embodiment of the presentinvention. Combined external defibrillator/pacer 8 includes a controlcircuit 10, an H-bridge 14, electrodes 15 a and 15 b, a charging circuit18, and an energy storage capacitor 24.

[0031] Defibrillator/pacer 8 is interconnected as follows. Controlcircuit 10 is connected to charging circuit 18 and H-bridge 14. Chargingcircuit 18 is connected to energy storage capacitor 24. H-bridge 14 isconnected to the electrodes of energy storage capacitor 24, and also toelectrodes 15 a and 15 b. Electrodes 15 a and 15 b are used toadminister defibrillation and pacing pulses transcutaneously to apatient. The operation of defibrillator/pacer 8 is described below inconjunction with FIG. 3.

[0032]FIG. 3 is a flow diagram illustrative of the operation ofdefibrillator/pacer 8. Referring to FIGS. 2 and 3, defibrillator/pacer 8operates as follows. In a step 70, defibrillator/pacer 8 decides whetherpacing or defibrillation pulses are appropriate for the patient.Alternatively, the user may make this determination. If pacing pulsesare appropriate, in a next step 71, defibrillator/pacer 8 is configuredto generate pacing pulses. For example, in this step, control circuit 10can control charging circuit 18 to charge energy storage capacitor 24 toa desired level for pacing. Then in a next step 72, defibrillator/pacer8 generates a pacing pulse using H-bridge 14. As described below,control circuit 10 may be configured to control H-bridge 14 to generatemonophasic or biphasic pacing pulses of different polarities.

[0033] In a next step 73, it is determined whether defibrillator/pacer 8should remain in the pacing mode. If defibrillator/pacer 8 is to remainin the pacing mode, the process returns to step 71. Otherwise, a step 74is performed in which external combined defibrillator/pacer 8 returns toa standby mode.

[0034] On the other hand, if in step 70, it is determined that adefibrillation pulse is appropriate, in a step 75, defibrillator/pacer 8is configured to generate a defibrillation pulse. In a next step 76,defibrillator/pacer 8 generates a defibrillation pulse using H-bridge14. The generation of a defibrillation pulse is described further belowin conjunction with FIG. 4. Then in a next step 77, the process returnsto the standby mode.

[0035]FIG. 4 is a more detailed block diagram of external combineddefibrillator/pacer 8 that is connected to a patient 16. Thedefibrillator includes a microprocessor 20 that is connected to energystorage capacitor 24 via charging circuit 18. It will be appreciated bythose skilled in the art that energy storage capacitor 24 may beimplemented with a multi-capacitor network (i.e., with capacitorsconnected in series and/or parallel). During the operation of thedefibrillator, microprocessor 20 controls charging circuit 18 using asignal on a control line 25 to charge energy storage capacitor 24 to adesired voltage level. To monitor the charging process, microprocessor20 is connected to a scaling circuit 22 by a pair of measurement lines47 and 48, and by a control line 49. Scaling circuit 22 is connected toenergy storage capacitor 24 by a bridge line 28, which connects to thenegative lead of energy storage capacitor 24, and by a line 30, whichconnects to the positive lead of the capacitor. A clock 21 is alsoconnected to microprocessor 20.

[0036] Scaling circuit 22 is used to step down the voltage across energystorage capacitor 24 to a range that may be monitored by microprocessor20. Scaling circuit 22 is described briefly below and in more detail inan application entitled “Method and Apparatus for Verifying theIntegrity of an Output Circuit Before and During Application of aDefibrillation Pulse”, U.S. patent application Ser. No. 08/811,834,filed Mar. 5, 1997, and incorporated herein by reference. Energy storagecapacitor 24 can be charged to a range of voltage levels, with theselected level depending on the patient and other parameters.Preferably, the size of energy storage capacitor 24 falls within a rangefrom 150 μF to 200 μF. In order to generate the necessary defibrillationpulse for external application to a patient, energy storage capacitor 24is charged to between 100 volts and 2,200 volts. To detect smallpercentage changes in the selected voltage level of energy storagecapacitor 24, scaling circuit 22 is adjustable to measure differentvoltage ranges. The adjusted output is measured by microprocessor 20 onmeasurement line 48.

[0037] After charging to a desired level, the energy stored in energystorage capacitor 24 may be delivered to patient 16 in the form of adefibrillation pulse. H-bridge 14 is provided to allow the controlledtransfer of energy from energy storage capacitor 24 to patient 16.H-bridge 14 is an output circuit that includes four switches 31, 32, 33,and 34. Each switch is connected in a leg of the output circuit that isarrayed in the form of an “H”. Switches 31 and 33 are coupled through aprotective component 27 to the positive lead of the energy storagecapacitor 24 by a bridge line 26. Protective component 27 limits thecurrent and voltage changes from energy storage capacitor 24, and hasboth inductive and resistive properties. Switches 32 and 34 are coupledto energy storage capacitor 24 by a bridge line 28. Patient 16 isconnected to the left side of H-bridge 14 by an apex line 17, and to theright side of H-bridge 14 by a sternum line 19. As depicted in FIG. 4,apex line 17 and sternum line 19 are connected to electrodes 15 a and 15b, respectively, by a patient isolation relay 35. Microprocessor 20 isconnected to switches 31, 32, 33, and 34 by control lines 42 a, 42 b, 42c, and 42 d, respectively, and to patient isolation relay 35 by controlline 36. A bypass circuit 40 is connected between bridge line 26 andapex line 17. Bypass circuit 40 is also connected to receive a controlsignal from microprocessor 20 through control line 42 e. Bypass circuit40 is implemented with a switch to bypass switch 33 when generatingpacing pulses, as described below.

[0038] Application of appropriate control signals by microprocessor 20over the control lines causes switches 31-34 to be appropriately openedand closed and bypass circuit 40 to be opened, thereby allowing H-bridge14 to conduct energy from energy storage capacitor 24 to patient 16 inthe form of a defibrillation pulse.

[0039] In a similar manner, microprocessor 20, through appropriateapplication of the control signals, causes switches 31-34 to beappropriately opened and closed and bypass circuit 40 to be closed,thereby allowing H-bridge 14 to conduct energy from storage capacitor 24to the patient in the form of a monophasic pacing pulse. Bypass circuit40 is needed to bypass switch SW33 because an SCR is used to implementthis switch. More specifically, at the size required to handle thedefibrillation energies, the SCR generally cannot be triggered by atypical pacing pulse current.

[0040] A preferred construction of H-bridge 14 is shown in FIG. 5.H-bridge 14 uses four output switches SW1-SW4 to conduct energy fromenergy storage capacitor 24 to patient 16. Switches SW1, SW3 and SW4 aresemiconductor switches, preferably silicon controlled rectifiers (SCRs).Switch SW2 is a series combination of switches SW2A and SW2B, preferablyboth insulated gate bipolar transistors (IGBTs). In this embodiment, theIGBTs are model IXHS1718 IGBTs available from IXYS, Santa Clara,California. Two model IXHS1718 IGBTs are used in “series” to withstandthe maximum voltage that may occur across switch SW2 in H-bridge 14 sothat the voltage across the entire switch SW2 is divided between the twoIGBTs. Alternatively, a single IGBT having a sufficient voltage ratingmay be used in the output circuit, where such an IGBT is available.Switches SW1-SW4 can be switched from an off (non-conducting) to an on(conducting) condition.

[0041] In the defibrillation mode, defibrillator/pacer 8 generates abiphasic defibrillation pulse for application to the patient 16.Initially, switches SW1-SW4 are opened. Charging of energy storagecapacitor 24 is started, and monitored by microprocessor 20 (FIG. 4).When energy storage capacitor 24 is charged to a selected energy leveland patient isolation relay 35 is closed, switches SW1 and SW2 areswitched on so as to connect energy storage capacitor 24 with apex line17 and sternum line 19 for the application of a first phase of adefibrillation pulse to patient 16. The stored energy travels from thepositive terminal of energy storage capacitor 24 on line 26, throughswitch SW1 and apex line 17, across patient 16, and back through sternumline 19 and switch SW2 to the negative terminal of energy storagecapacitor 24 on line 28. The first phase of the biphasic pulse istherefore a positive pulse from the apex to the sternum of patient 16.

[0042] Before energy storage capacitor 24 is completely discharged,switch SW2 is biased off to prepare for the application of the secondphase of the biphasic pulse. Once switch SW2 is biased off, switch SW1will also become non-conducting because the voltage across the SCR fallsto zero.

[0043] After the end of the first phase of the biphasic defibrillationpulse, switches SW3 and SW4 are switched on to start the second phase ofthe biphasic pulse. Switches SW3 and SW4 provide a current path to applya negative defibrillation pulse to patient 16. The energy travels fromthe positive terminal of energy storage capacitor 24 on line 26, throughswitch SW3 and sternum line 19, across patient 16, and back through apexline 17 and switch SW4 to the negative terminal of energy storagecapacitor 24 on line 28. The polarity of the second phase of thedefibrillation pulse is therefore opposite in polarity to the firstphase of the biphasic pulse. The end of the second phase of the biphasicpulse is truncated by switching on switch SW1 to provide a shorted pathfor the remainder of the capacitor energy through switches SW1 and SW4.After energy storage capacitor 24 is discharged, switches SW1-SW4 areswitched off. Patient isolation relay 35 is then opened. Energy storagecapacitor 24 may then be recharged to prepare defibrillator/pacer 8 toapply another defibrillation pulse or to apply pacing pulses.

[0044] As described above, the four output switches SW1-SW4 can beswitched from an off (nonconducting) state to an on (conducting) stateby application of appropriate control signals on control lines 42 a, 42b, 42 c, and 42 d. In order to allow the SCRs and IGBTs to switch thehigh voltages in an external defibrillator, special switch drivingcircuits 51, 52, 53 and 54 are coupled to switches SW1-SW4,respectively. Control lines 42 a, 42 b, 42 c, and 42 d are connected toswitch driving circuits 51, 52, 53, and 54, to allow microprocessor 20to control the state of the switches.

[0045] Switch driving circuits 51, 53 and 54 are identical. For purposesof this description, therefore, only the construction and operation ofswitch driving circuit 51 will be described. Those skilled in the artwill recognize that switch driving circuits 53 and 54 operate in asimilar manner.

[0046] Switch driving circuit 51 includes a control switch SW11,resistors R11, R12, and R13, a capacitor C11, a diode D11 and ahigh-voltage transformer T11. Resistor R11 is connected between thepositive voltage supply V′+ and the dotted end of the primary winding oftransformer T11, and capacitor C11 is connected between ground and thedotted end of the primary winding of transformer T11. Resistor R12 isconnected between the non-dotted end of the primary winding oftransformer T11 and the drain of control switch SW11. Resistors R11 andR12 and capacitor C11 limit and shape the current and voltage waveformsacross the primary winding of transformer T11. The source of controlswitch SW11 is connected to ground, and the gate of control switch SW11is connected to control line 42 a.

[0047] On the secondary winding side of transformer T11, the anode ofdiode D11 is connected to the dotted end of the secondary winding oftransformer T11, and the cathode of diode D11 is connected to the gateof SCR switch SW1. Resistor R13 is connected between the cathode ofdiode D11 and the non-dotted end of the secondary winding of transformerT11. The non-dotted end of the secondary winding of transformer T11 isconnected to the cathode of SCR switch SW1.

[0048] To turn on switch SW1, an oscillating control signal is providedon control line 42 a. In this embodiment, the oscillating control signalis a pulse train. The pulse train control signal repeatedly turnscontrol switch SW11 on and off, producing a changing voltage across theprimary winding of transformer T11. The voltage is stepped down bytransformer T11 and rectified by diode D11 before being applied to thegate of SCR switch SW1. In a preferred embodiment, a 10% duty cyclepulse train on the control line 42 a has been found to be adequate tomaintain SCR switch SW1 in a conducting state. As long as the controlsignal is applied to the switch driving circuit 51, the switch SWI willremain in the conducting state. The switch SW1 remains in the conductingstate even when conducting relatively low defibrillation currents. Inone embodiment, the SCR switches can conduct currents as low as 90 mA.As is well known, once triggered or latched on, an SCR generally remainsin the conducting state until the current through the SCR drops below aminimum level, even if the gate voltage of the SCR is grounded. Thus,when the current through the SCR switch would not be 90 mA or greater,the SCR would not conduct. Thus, SCRs are generally not practical forpacing applications.

[0049] A different switch driving circuit is required to turn on theIGBT switches of switch SW2. Switch driving circuit 52 includes acapacitor C21, a transformer T21, and two identical switch drivingcircuits 52A and 52B, each circuit corresponding to one of the IGBTs. Onthe primary winding side of transformer T21, capacitor C21 is connectedbetween control line 42 b and the non-dotted end of the primary windingof transformer T2 1. The dotted end of the primary winding oftransformer T21 is grounded.

[0050] Transformer T21 has two secondary windings T21A and T21B, one foreach of switch driving circuits 52A and 52B. Switch driving circuits 52Aand 52B are identical, and therefore only the construction and operationof switch driving circuit 52A will be described. Switch driving circuit52A includes diodes D21, D22, D23, and D24, Zener diode ZD21, capacitorsC22, C23, C24, and C25, resistors R21, R22, R23, and R24, a PNP switchSW23, and an SCR switch SW22.

[0051] The anodes of diodes D21, D22, and D23 are connected to thenon-dotted end of secondary winding T21A of transformer T21. Thecathodes of diodes D21 and D22 are connected to the gate of IGBT switchSW2A. Resistor R21 and capacitor C22 are connected between the dottedend of secondary winding T21A of transformer T21 and the cathode ofdiode D23. The anode of SCR switch SW22 and the cathode of Zener diodeZD21 are connected to the gate of IGBT switch SW2A. The cathode of SCRswitch SW22 and the anode of Zener diode ZD21 are connected to thedotted end of secondary winding T21A of transformer T21, and also to theemitter of IGBT switch SW2A.

[0052] Resistor R23 and capacitor C24 are connected between the gate ofIGBT switch SW2A and the emitter of PNP switch SW23. Resistor R24 andcapacitor C25 are connected between the emitter of PNP switch SW23 andthe dotted end of secondary winding T21A of transformer T21. The gate ofSCR switch SW22 is connected to the collector of PNP switch SW23.Resistor R22 is connected between the collector of PNP switch SW23 andthe dotted end of secondary winding T21A of transformer T21. CapacitorC23 is connected between the emitter and the base of PNP switch SW23.The anode of diode D24 is connected to the base of PNP switch SW23, andthe cathode of diode D24 is connected to the cathode of diode D23.

[0053] To turn on IGBT switch SW2A, an oscillating control signal isprovided on control line 42 b. In this embodiment, the oscillatingcontrol signal is a pulse train. The pulse train control signal isstepped up in voltage by transformer T21 and applied to the input ofswitch driving circuit 52A. During a positive pulse of the controlsignal on control line 42 b, diodes D21 and D22 rectify the current thattravels through secondary winding T21A to charge capacitors C24 and C25.As will be discussed in more detail below, some current also travelsthrough diode D23 to charge capacitor C22.

[0054] Capacitor C21 limits the current in the primary winding oftransformer T21, which correspondingly limits the current in secondarywinding T21A. The secondary winding current determines the charging timeof the capacitors C24 and C25. Since the voltage across capacitors C24and C25 is also the voltage on the gate of IGBT switch SW2A, a slowaccumulation of voltage on capacitors C24 and C25 therefore results in aslow turn on of IGBT switch SW2A. The charging current is selected sothat IGBT switch SW2A is turned on relatively slowly when compared tothe fast turn on of SCR switches SW1, SW3, and SW4. A slow turn-on forIGBT switch SW2A is desirable because the IGBT switches are on the sameside of H-bridge 14 as SCR switch SW3. SCR switch SW3 is controlled bythe control signal on control line 42 c, but due to the nature of SCRswitches, the SCR switch may be accidentally turned on regardless of thesignal on control line 42 c if a rapid voltage change occurs across SCRswitch SW3. If IGBT switches SW2A and SW2B were therefore turned on tooquickly, the resulting rate of change of the voltage across SCR switchSW3 might cause it to turn on accidentally.

[0055] Zener diode ZD21 protects IGBT switch SW2A by regulating themaximum voltage across capacitors C24 and C25. Without Zener diode ZD21, the voltage on the gate of IGBT switch SW2A would rise to a levelthat would damage IGBT switch SW2A.

[0056] Also during the positive pulse of the pulse train control signalon control line 42 b, diode D23 rectifies the current that travelsthrough secondary winding T21A to charge capacitor C22. The charge oncapacitor C22, which is replenished on each positive pulse of the pulsetrain control signal, maintains the voltage across the base of PNPswitch SW23 above the turn-on level for the PNP switch. PNP switch SW23turns on if the base voltage on the switch drops below a thresholdlevel. As will be described below, PNP switch SW23 is only turned onwhen IGBT switch SW2A is to be turned off. Capacitor C23 and diode D24are also provided to prevent PNP switch SW23 from turning on. CapacitorC23 serves as a high frequency filter to prevent the high frequencydriving pulses of switch driving circuit 52A from causing PNP switchSW23 to spuriously turn on. Diode D24 prevents a large negativebase-emitter voltage from occurring which could cause PNP switch SW23 toenter reverse breakdown.

[0057] Since some discharging of capacitor C22 occurs through resistorR21 between positive pulses of the control signal on control line 42 b,resistor R21 must be large enough to limit the discharging current flowfrom capacitor C22 between the pulses. Limiting the current flowprevents the voltage on capacitor C22 from dropping below the thresholdlevel sufficient to turn on PNP switch SW23 between pulses of thecontrol signal. Then, during a positive pulse of the pulse train controlsignal on control line 42 b, the charging of capacitor C22 must besufficient to counteract the discharging that occurred since theprevious positive pulse so as to return capacitor C22 to its fullycharged level by the end of the positive pulse.

[0058] In the preferred embodiment, a 2 MHz pulse train control signalwith a 25% duty cycle on the control line 42 b has been found to beadequate to maintain the conducting state of IGBT switches SW2A andSW2B. The switches will remain conducting as long as the control signalis present, and regardless of the current flowing through the switches.Conversely, when the control signal is not present, IGBT switches SW2Aand SW2B will be non-conductive.

[0059] The maximum current that may generally occur in H-bridge 14results from the undesirable situation where a user ofdefibrillator/pacer 8 places the two shock paddles directly in contactwith one another. When this happens, a short circuit is created betweenapex line 17 and sternum line 19. During a short circuit, a briefcurrent of up to 400 amps can result. In this embodiment, to accommodatethe short circuit current without damaging IGBT switches SW2A and SW2B,IGBT switches SW2A and SW2B are biased by a thirty volt gate voltage.Biasing the IGBTs at this voltage level is successful since the IGBTswitches are used in a pulsed manner. If IGBT switches SW2A and SW2Bwere driven continuously for long periods of time with thirty volts ontheir gates, they might be damaged, but in H-bridge 14, they are onlydriven at this level for very brief intervals.

[0060] In contrast to the slow turn-on of IGBT switches SW2A and SW2B,the turnoff of the IGBT switches is performed relatively quickly. TheIGBT switches may be quickly turned off because at turn-off there is noconcern that the sensitive SCR switches will accidentally turn on. Inaddition, a fast turn-off is desirable to reduce the time that an IGBTswitch would be subjected to a high voltage if one of the IGBT switchesis inadvertently turned off before the other.

[0061] IGBT switches SW2A and SW2B are turned off when the pulse traincontrol signal on control line 42 b is removed. Once positive voltagepulses are no longer being induced in the secondary windings oftransformer T21, driving circuits 52A and 52B begin the turn-offprocess. Again, the turn-off process will only be described with respectto driving circuit 52A since the circuits are essentially identical.

[0062] During the turn-off process, capacitor C22 begins dischargingthrough resistor R21. Since the RC time constant of capacitor C22 andresistor R21 is much smaller than the RC time constant of capacitors C24and C25 and resistors R23 and R24, the discharging of capacitor C22occurs much more quickly than the discharging of capacitors C24 and C25.When the voltage on capacitor C22 drops below a threshold voltage level,PNP switch SW23 is turned on. The threshold voltage level is equivalentto the base turn-on voltage of PNP switch SW23, plus the voltage dropacross diode D24. Once PNP switch SW23 is turned on, discharge currentfrom capacitor C25 begins to flow through the switch. As the currentincreases, the voltage across resistor R22 correspondingly increases.When the voltage across resistor R22 reaches a sufficient voltage level,SCR switch SW22 is turned on, providing a shorted path for the remainderof the energy stored in capacitors C24 and C25. The rapid discharge ofcapacitors C24 and C25 causes a corresponding rapid drop in the gatevoltage of IGBT switch SW2A, quickly turning off the switch. ResistorsR23 and R24 are provided across capacitors C24 and C25 to control thevoltage division across the capacitors.

[0063] It will be appreciated that special driving circuits 52A and 52Ballow the IGBTs to be used in external defibrillator/pacer 8 whereextremely high voltages must be switched in the presence of SCRs. Thedriving circuits minimize the number of components required to switch adefibrillation pulse of 200 or more joules. In addition to conductinghigh currents associated with high-energy defibrillation pulses, theIGBTs are also able to conduct very low currents that are associatedwith defibrillation pulses of less than 50 joules.

[0064] As shown in FIG. 5, each switch SW1-SW4 is also connected inparallel with a switch protection circuit 61, 62, 63, and 64,respectively. The switch protection circuits are designed to preventspurious voltage spikes from damaging the switches in H-bridge 14.Switch protection circuits 61, 63 and 64 are identical and thereforeonly the construction and operation of switch protection circuit 61 willbe described. Switch protection circuit 61 includes a diode D12. Thecathode of diode D12 is connected to the anode of SCR switch SW1, andthe anode of diode D12 is connected to the cathode of SCR switch SW1.Diode D12 protects SCR switch SW1 against negative inductive spikes thatmay occur due to cable or load inductance.

[0065] Switch protection circuit 62 includes two identical switchprotection circuits 62A and 62B, which protect IGBT switches SW2A andSW2B, respectively. Since switch protection circuits 62A and 62B areessentially identical, only the construction and operation of switchprotection circuit 62A will be described. Switch protection circuit 62Aincludes a diode D24 and a resistor R23. Resistor R23 is connectedbetween the collector and the emitter of IGBT switch SW2A. The cathodeof diode D24 is connected to the collector of IGBT switch SW2A, and theanode of diode D24 is connected to the emitter of IGBT switch SW2A.

[0066] Diode D24 operates similarly to diode D12 as described above inthat it protects IGBT switch SW2A against negative inductive spikes.Resistor R23 (in conjunction with resistor R23′) ensures that thevoltage across the two IGBT switches SW2A and SW2B is equally dividedwhen H-bridge 14 is at rest. Dividing the voltage across IGBT switchesSW2A and SW2B is important due to the limitations of present IGBTtechnology, which limits the rating of each IGBT switch to 1200V. In asystem where the total maximum voltage is 2200V, the maximum voltageratings are therefore obeyed by dividing the maximum voltage across eachIGBT switch.

[0067] Additional protection to the switches is provided by protectivecomponent 27, which has both inductive and resistive properties. In oneembodiment, protective circuit 27 is implemented with coil of resistancewire that provides an inductive resistance. Protective component 27limits the rate of change of the voltage across, and current flow to,SCR switches SW1, SW3, and SW4. Too high of a rate of change of thevoltage across an SCR switch is undesirable because it can cause the SCRswitch to inadvertently turn on. For example, since SCR switches SW1 andSW4 are on the same side of H-bridge 14, any time SCR switch SW4 isabruptly turned on, a rapid voltage change may also result across SCRswitch SWI. To prevent rapid voltage changes, protective component 27reduces the rate of change of the voltage across SCR switch SW1 when SCRswitch SW4 is turned on. Also, too high of a current flow can damage theswitches SW1, SW3 and SW4, and protective component 27 limits thecurrent flow in H-bridge 14. The use of protective component 27therefore reduces the need for additional protective components thatwould otherwise need to be coupled to switches SW1, SW3 and SW4.

[0068] It will be appreciated that a great advantage of H-bridge 14described above is that it allows external defibrillator/pacer 8 togenerate and apply a high-energy biphasic waveform to a patient. Forprior defibrillators providing a monophasic waveform, the standardenergy level in the industry for the discharge has been greater than 200joules. The above described circuit allows the same amount of energy(more than 200 joules) to be delivered to the patient in a biphasicwaveform, thereby resulting in a greater certainty of defibrillationeffectiveness for a broader range of patients. At the same time, thecircuit incorporates special driving circuitry to allow even very lowenergy biphasic waveforms (less than fifty joules) to be delivered tothe patient.

[0069] The above described defibrillation mode operation is similar tothe operation of the external defibrillator circuit disclosed inco-pending and commonly assigned U.S. patent application Ser. No.08/811,833 filed Mar. 5, 1997, entitled “H-Bridge Circuit For GeneratingA High-Energy Biphasic Waveform In An External Defibrillator” by J. L.Sullivan et al. In a manner similar to the defibrillator disclosed inthe Ser. No. 08/811,833 application, this embodiment ofdefibrillator/pacer 8 generates a biphasic defibrillation pulse with apositive first phase and a negative second phase (measured from apexline 17 to sternum line 19).

[0070] The operation of this embodiment of defibrillator/pacer 8 in thepacing mode is represented by the flow diagram of FIG. 6. In view of thepresent disclosure, those skilled in the art can implement, withoutundue experimentation, a suitable software or firmware program to beexecuted by microprocessor 20 of control circuit 10 to perform functionsrepresented in FIG. 6. Referring to FIGS. 4, 5, and 6,defibrillator/pacer 8 operates as follows. Initially, switches SW1-SW4and bypass circuit 40 are opened. Once configured in the pacing mode, ina step 80, control circuit 10 determines whether to generate a pacingpulse. If no pacing pulse is to be generated, the process returns to thestandby mode in a step 81. However, if a pacing pulse is to begenerated, in a next step 82, control circuit 10 causes charging circuit18 to charge energy storage capacitor 24 to a capacitor voltage level ofabout twenty-five to three hundred volts. The capacitor voltage levelwould vary according to desired pacing current for the pending pacingpulse, as described below.

[0071] In a next step 83, control circuit 10 causes a relay K4 in bypasscircuit 40 to close. In one embodiment, relay K4 is implemented with twomodel RTD19005 relays available from Potter-Bromfield, Princeton,Indiana. When relay K4 is closed, a conductive path is formed betweenthe positive electrode of energy storage capacitor 24, through line 28,through a resistor R4, through relay K4 and to apex line 19. As aresult, switch SW3 is bypassed. Resistor R4 has a value of about fivehundred ohms to help limit the short circuit current if the paddlestouch.

[0072] In a next step 85, control circuit 10 pulses on IGBTs SW2A andSW2B to allow a monophasic pacing pulse to be applied to patient 16. Inthis embodiment, control circuit 10 controls the duration that IGBTsSW2A and SW2B are pulsed on to be equal to the desired duration of themonophasic pacing pulse. For a given capacitance value of energy storagecapacitor 24, the droop of the pacing pulses would vary according topatient impedance.

[0073] In a step 87, control circuit 10 measures the pacing current asthe pacing pulse is being applied to patient 16. In one embodiment, thepacing current is measured by monitoring the voltage drop acrossresistor R4. Alternatively, the pacing current can be measured bymonitoring the change in voltage of energy storage capacitor 24. In anext step 89, control circuit 20 determines the capacitor voltage levelacross energy storage capacitor 24 that is required for the next pacingpulse. In this embodiment, the current of the next pacing pulse isincreased by about 5 mA. That is, during pacing, the pacing current isincreased in 5 mA increments until the pacing pulses are of sufficientstrength to cause the heart muscle to contract or to a maximum of about200 mA. The process then returns to step 80 to generate the next pacingpulse.

[0074] The pacing pulses generated by this embodiment are positive(measured from apex line 17 to sternum line 19). However, because thepolarity of a pacing pulse affects pacing capture threshold, the generalpractice is to generate pacing pulses that are negative. Negative pacingpulses can be generated by simply by switching apex and sternum lines 17and 19. FIG. 7 shows waveforms representing the waveforms generated bythis alternative embodiment. Waveform 90 represents a biphasicdefibrillation pulse generated by this alternative, while waveform 91represents a monophasic pacing pulse sequence. Due to the switching ofapex and sternum lines 17 and 19 in this embodiment, defibrillationwaveform 90 has a negative first phase 93 and a positive second phase94. Pacing waveform 91 has negative monophasic pulses 96, 97 and so on.Although not shown in FIG. 7, the biphasic waveforms may have a smalldelay portion of about zero volts between the first and second phases.

[0075] Although this alternative embodiment generates negative pacingpulses, the polarity of the defibrillation pulse phases is reversed.Current research tends to show that the polarity of monophasicdefibrillation pulses is not significant. This finding may also apply tobiphasic defibrillation pulses. Accordingly, this embodiment may bepractical for use in the field.

[0076]FIG. 8 is a block diagram of an alternative embodiment of H-bridge14 that is advantageously used to generate defibrillation and pacingpulses with opposite polarity. This embodiment is similar to theembodiment of FIG. 5 except that the embodiment of FIG. 8 uses an IGBTto implement switch SW4. Thus, switches SW1, SW2 and SW3 and drivingcircuits 51, 52 and 53 of FIG. 8 are implemented as in the embodiment ofFIG. 5. Because switch SW4 is an IGBT switch, switch driving circuit 54Ais essentially identical to switch driving circuit 52 used for drivingIGBT switch SW2. In addition, the embodiment of FIG. 8 uses a switchdriving circuit 54A and a current source circuit 50 instead of theswitch driving circuit 54 and bypass circuit 40 of FIG. 5.

[0077] In one embodiment, current source circuit 50 is implemented withan IGBT circuit 70. An IGBT circuit is used because the circuit must bestrong enough to withstand the relatively large voltages used in thedefibrillation mode. IGBT circuit 70 is connected to line 26 andresistor R4. In this embodiment, resistor R4 has a value of about 10 Ω.Resistor R4 is also connected to sternum line 19. When conductive, IGBTcircuit 70 provides a current path from line 26 to resistor R4 and on tosternum line 19. A switch driving circuit 71 is connected to controlline 42 e and the gate of IGBT circuit 70. Switch driving circuit 71turns IGBT circuit 70 off and on in response to a control signal fromcontrol circuit 10 received on control line 42 e. In addition, in thisembodiment, control circuit 10 (FIG. 4) is connected to monitor thevoltage across resistor R4 via lines 72 and 73 during the pacing mode.Control circuit 10 (FIG. 4) is configured to provide the control signalover control line 42 e to cause switch driving circuit 71 to operateIGBT circuit 70 in the linear region so that the current conducted byIGBT circuit 70 can be adjusted to a desired level. This technique isreferred to herein as constant current pacing because the current can bemaintained at a constant peak level even if patient impedance changesbetween pulses and is not adjusted with regard to the energy dischargedby energy storage capacitor 24. Switch driving circuit 71 is describedfurther below in conjunction with FIG. 10.

[0078] In the defibrillation mode, current source circuit 50 is turnedoff and switches SW1 and SW2 are turned on to generate the first phaseof the biphasic defibrillation pulse. As shown in FIG. 9, this switchingsequence generates biphasic defibrillation waveform 90′ with a firstphase 93′ that is positive. To generate the second phase, switch SW2 isturned off. As described above for the embodiment of FIG. 5, turning offswitch SW2 causes switch SW1 to turn off. Then switches SW3 and SW4 areturned on. As a result, second phase 93′ in FIG. 9 is negative. SwitchesSW1, SW2 and SW3 are turned off and off in essentially the same manneras described above for the embodiment of FIG. 5. Switch SW4 is turnedoff and on in essentially the same manner as switch SW2.

[0079] In the pacing mode, current source circuit 50 and switch SW4 areturned on. Current source circuit 50 is controlled by control circuit 10(FIG. 4) to provide a desired level of pacing current to sternum line19. As previously described, the pacing current is typically increasedby about 5 mA with each successive pacing pulse until the pulses are ofsufficient strength to cause the heart muscle to contract. Controlcircuit 10 (FIG. 4) controls switch SW4 to be turned on for the desiredduration of a pacing pulse. Because switch SW4 is implemented with anIGBT in this embodiment (in contrast to the SCR used in FIG. 5), switchSW4 is conductive even for the relatively small currents of the pacingpulses. Thus, as shown in FIG. 9, pulses 96 and 97 of waveform 91 arenegative pulses. The embodiment of FIG. 8 advantageously allows forgeneration of biphasic defibrillation pulses with a positive first phaseand negative second phase, while also allowing generation of negativemonophasic pacing pulses.

[0080] Alternatively, control circuit 10 (FIG. 4) may be configured toadjust the voltage across energy storage capacitor 24 instead of thecurrent provided by current source circuit 50. In this alternativeembodiment, control circuit 10 (FIG. 4) would estimate the capacitorvoltage required to produce the desired pacing current for the nextpacing pulse. In addition, current source circuit 50 would be operatedas a switch. Thus, switch driving circuit 71 can be identical to switchdriving circuit 52 (FIG. 5). In this alternative embodiment, the valueof resistor R4 would be increased to about 500 Ω or greater to providebetter current regulation. In another embodiment of this alternativecapacitor voltage adjustment technique, current source circuit 50 may bereplaced with bypass circuit 40 (FIG. 5) that uses a relativelyinexpensive relay circuit instead of a relatively costly IGBT circuit.The differences between the current source regulation technique and thiscapacitor voltage regulation technique are more clearly set forth inFIG. 11.

[0081]FIG. 10 is a diagram illustrative of one embodiment of IGBT switchdriving circuit 71 for biasing an IGBT to operate in the linear regionso as to control the current conducted by the IGBT. In this embodiment,switch driving circuit 71 includes an operational amplifier 75,resistors 76 and 77, a capacitor 78 and a diode 79.

[0082] Switch driving circuit 71 is interconnected as follows.Operational amplifier 75 has its non-inverting input lead connected tocontrol line 42 e. The output lead of operational amplifier 75 isconnected to one terminal of resistor 76. The other terminal of resistor76 is connected to the gate of IGBT 70. Resistor 77 is connected betweenthe inverting input lead of operational amplifier 75 and the drain ofIGBT 70. Capacitor 78 and diode 79 are connected between the output leadand the inverting input lead of operational amplifier 75, with diode 79being connected to allow current to flow from the inverting input leadto the output lead of operational amplifier 75.

[0083] Switch driving circuit 71 uses a feedback scheme to control IGBT70 to operate in the linear region to achieve a desired output current.Control circuit 10 (FIG. 4) monitors the current outputted by IGBT 70through resistor R4 as previously described and adjusts the voltagelevel of the control signal at control line 42 e to achieve the desiredcurrent level. Capacitor 78 causes switch driving circuit 71 to functionin a manner similar to an integrator, with its input signal being thevoltage at the drain of IGBT 70. Thus, when the voltage level at controlline 42 e is zero, the output voltage of operational amplifier 75 isalso zero, causing IGBT 70 to be non-conductive.

[0084] When the voltage at control line 42 e is positive relative to thevoltage at the inverting input lead of operational amplifier 75, the“integrator” operates to increase the voltage at its output lead, whichin turn causes IGBT 70 to be more conductive and pull up the voltage atits drain. Because this drain voltage is fed back to the inverting inputlead of operational amplifier 75, the “integrator” only increases itsoutput voltage until the drain voltage is substantially equal to thevoltage at the non-inverting input lead. That is, the “virtual ground”effect of operational amplifiers causes the “integrator” to quicklydrive the drain voltage to be equal to the voltage at control line 42 e.

[0085] In a symmetrically opposite manner, when the voltage at controlline 42 e is negative relative to the voltage at the inverting inputlead of operational amplifier 75, the “integrator” operates to decreasethe voltage at its output lead, which in turn causes IGBT 75 to becomeless conductive and quickly drive the drain voltage of IGBT 70 to besubstantially equal to the voltage at control line 42 e. Diode 79 helpsto prevent the voltage at the drain of IGBT 70 from being above thevoltage at the gate of IGBT 70 by more than a diode threshold voltage.

[0086] Thus, when the control circuit 10 (FIG. 4) wishes to increase theoutput current of IGBT 70, control circuit 10 causes the voltage levelat control line 42 e to increase. As described above, increasing thevoltage at control line 42 e causes the output current of IGBT 75 toincrease, which is then detected by control circuit 10 (FIG. 4) inmonitoring the voltage across resistor R4. When the output currentreaches the desired level, control circuit 10 (FIG. 4) can then stopincreasing the voltage at control line 42 e. Conversely, to decrease theoutput current of IGBT 70, control circuit 10 (FIG. 4) causes thevoltage level at control line 42 e to decrease, thereby causing theoutput current of IGBT 70 to decrease. When the output current reachesthe desired level, control circuit 10 (FIG. 4) can then stop decreasingthe voltage at control line 42 e.

[0087]FIG. 11 is a flow diagram summarizing the operation of theembodiment of FIG. 8 in generating pacing pulses for both capacitorvoltage regulation and current source regulation. Steps 80-82 are asdescribed above in conjunction with FIG. 6. In the capacitor voltageregulation technique, control circuit 10 (FIG. 4) performs steps 83′,85′, 87′ and 89′, which are essentially the same as steps 83, 85, 87 and89 described above in conjunction with FIG. 6 (except that currentsource circuit 50 is used instead of bypass circuit 40).

[0088] In the current regulation technique, following step 82, controlcircuit 10 (FIG. 4) controls switch driving circuit 71 to activatecurrent source circuit 50 to provide the desired current for the pendingpacing pulse in a step 84. Then steps 85′ and 87′ are performed as inthe capacitor voltage regulation technique. In a next step 88, controlcircuit 10 (FIG. 4) determines the control signal adjustments needed toproduce the desired pacing current for the next pulse. As describedearlier, the pacing current is typically increased in 5 mA incrementsuntil a maximum level is reached.

[0089]FIG. 12 is a diagram illustrating an embodiment of H-bridge 14that has four IGBT legs and no SCR legs that can be advantageously usedto generate biphasic pacing and biphasic defibrillation pulses using aconstant energy technique. This embodiment is similar to the embodimentof FIG. 8, except that current source circuit 50 is omitted and switchesSW1 and SW3 are IGBT switches essentially identical to IGBT switch SW2.In addition, instead of switch driving circuits 51 and 53 (FIG. 5) forSCR switches, this embodiment includes switch driving circuits 51A and53A that are essentially identical to switch driving circuit 71 (FIG.10). In particular, switch driving circuits 51 A and 53A would includecircuitry similar to circuit 71 (FIG. 10) to operate the IGBTs in thelinear region to control the pacing current. To generate bothdefibrillation and pacing pulses, switches SW1-SW4 are turned off and onin the same sequence described above in conjunction with FIG. 8 togenerate defibrillation pulses. However, when generating pacing pulses,control circuit 10 (FIG. 4) controls the current level for eachsubsequent pacing pulse by operating the IGBTs of switches SW1 and SW3in the linear region. The use of IGBT switches in all of the legs ofH-bridge 14 allow conduction of the relatively small currents used inpacing pulses, while withstanding the relatively high current levelsused in defibrillation pulses.

[0090] To generate a biphasic pulse with a positive first phase and anegative second phase, switches SW1 and SW2 would be turned on duringthe first phase while switches SW3 and SW4 are turned off. The negativesecond phase would be generated by turning off switches SW1 and SW2 andturning on switches SW3 and SW4. Conversely, to generate biphasic pulsesof the opposite polarity (i.e., with a negative first phase), during thefirst phase, switches SW3 and SW4 would be turned on while switches SW1and SW2 are turned off. The positive second phase would be generated byturning on switches SW1 and SW2 and turning off switches SW3 and SW4.

[0091]FIG. 13 illustrates two defibrillation waveforms (i.e., waveforms90 and 90′) and two pacing waveforms (i.e., waveforms 91′ and 91″) thatcan be generated by using this embodiment of H-bridge 14 (FIG. 12). Forexample, this embodiment of H-bridge 14 can generate biphasicdefibrillation waveform 90′ having a positive first phase 93′ and anegative second phase 94′. H-bridge 14 (FIG. 12) can also generatebiphasic defibrillation waveform 90, which has a negative first phase 93and a positive second phase 94. H-bridge 14 (FIG. 12) can also generatebiphasic pacing waveform 91′ in which each of the biphasic pacing pulseshas a negative first phase 98 and a positive second phase 99. H-bridge14 (FIG. 12) can also generate biphasic pacing waveform 91″ in whicheach of the biphasic pacing pulses has a negative first phase 98′ and apositive second phase 99′.

[0092] Although not illustrated in FIG. 13, those skilled in the artwill appreciate that H-bridge 14 (FIG. 12) can also generate monophasicwaveforms or even other multiphasic waveforms with appropriate controlof switches SW1-SW4. In light of this disclosure, those skilled in theart can, without undue experimentation, provide a suitable software orfirmware program that microprocessor 20 of control circuit 10 (FIG. 4)can execute to generate the appropriate switch control signals.

[0093]FIG. 14 is a diagram illustrating still another embodiment ofH-bridge 14. This embodiment is similar to the embodiment of FIG. 12,except that this embodiment includes a current sensing circuit 56connected between energy storage capacitor 24 and line 26 that suppliescurrent from energy storage capacitor 24 to switches SW1-SW4. Thisembodiment is advantageously used to generate constant current pacingpulses by linear operation of switches SW2 and SW4 (or, alternatively,switches SW1 and SW3). Consequently, the switch driving circuitsconnected to switches SW2 and SW4 (or switches SW1 and SW3 in thealternative embodiment) are essentially identical to switch drivingcircuit 71 (FIG. 10).

[0094] During the defibrillation mode, switches SW1-SW4 are controlledto operate as switches (i.e., not as current sources). To generate abiphasic defibrillation pulse, control circuit 10 (FIG. 4) is configuredto turn switches SW1-SW4 off and on as described above for theembodiment of FIG. 12, without regard to the current sensed by currentsensing circuit 56.

[0095] However, in operation during the pacing mode, current sensingcircuit 56 monitors the current flowing from energy storage capacitor 24to line 26 and generates on a line 57 a current sense signal indicativeof the current level. Control circuit 10 (FIG. 4) is connected to line57 and, based on the detected current level, adjusts the control signalson lines 42 d and 42 b so that IGBT switches SW2 and SW4 conduct thedesired level of current for each phase the pacing pulse. This can bedone “on-the-fly” by control circuit 10. Of course, in the alternativeembodiment in which switches SW1 and SW3 are operated in the linearregion, control circuit 10 (FIG. 4) would adjust the control signal onlines 42 a and 42 c so that IGBT switches SW1 and SW3 conduct thedesired level of current for each phase of the pacing pulse.

[0096]FIG. 15 illustrates one embodiment of current sensing circuit 56.In this embodiment, current sensing circuit 56 includes a transformerT56, a resistor 58 and an amplifier 59. The primary winding oftransformer T56 is connected to conduct the current flowing from energystorage capacitor 24 to line 26. Thus, as current flows in the primarywinding of transformer T56, a proportional current flows in thesecondary winding of transformer T56 and through resistor 58. The inputleads of amplifier 59 are connected on either side of resistor 58 sothat amplifier 59 will generate on line 57 an output signal having alevel that is a function of the voltage drop across resistor 58. Giventhe known characteristics of amplifier 59 and transformer T56, controlcircuit 10 (FIG. 4) can generate appropriate control signals to controlthe current conducted by switches SW2 and SW4 to the desired pacingcurrent levels.

[0097] While the preferred embodiment of the invention has beenillustrated and described, it will be apparent that various changes canbe made therein without departing from the spirit and scope of theinvention. For example, control lines 42 c and 42 d and control switchesSW31 and SW41 could be replaced by a single control line and controlswitch to activate switch driving circuits 53 and 54. Also, while thepreferred construction for switches 31, 32, 33, and 34 is describedabove, it will be appreciated that other switch constructions may beenvisioned, such as replacing switch 32 with a single IGBT of sufficientstand-off voltage. Or, additional semiconductor switches may beincorporated in each leg to reduce the voltage that must be switched byeach switch. To minimize the size and weight of the resulting H-bridgeoutput circuit, however, the construction described above is preferable.Consequently, within the scope of the appended claims, it will beappreciated that the invention can be practiced otherwise than asspecifically described herein. Further, although an H-bridgeconfiguration is described -for-the energy transfer circuit, other typesof energy transfer circuits may be used.

We claim:
 1. A circuit for use in an external unit that generates a defibrillation pulse in a defibrillation mode and a pacing pulse in a pacing mode, the circuit comprising: an energy storage capacitor having a first electrode and a second electrode; a charging circuit coupled to the energy storage capacitor, wherein the charging circuit is configured to charge the energy storage capacitor; an energy transfer circuit coupled to the energy storage capacitor, the Energy transfer having a first output lead and a second output lead, wherein the energy transfer is configured to selectively electrically couple the first and second electrodes of the energy storage capacitor to the first and second output leads; and a control circuit coupled to the charging circuit and the energy transfer circuit, wherein the control circuit is configured to cause the charging circuit to charge the energy storage capacitor to a predetermined level and, when the energy storage capacitor is charged, to control the energy transfer circuit to couple the first and second electrodes of the energy storage capacitor to the first and second output leads of the energy transfer circuit so that the energy transfer circuit provides: during the defibrillation mode, a defibrillation pulse at the first and second output leads using energy stored in the energy storage capacitor, and during the pacing mode, a pacing pulse at the first and second output leads using energy stored in the energy storage capacitor.
 2. The circuit of claim 1 wherein the energy transfer circuit comprises four legs, a first leg of the energy transfer circuit including a first IGBT switch circuit and a second, a third and fourth leg of the energy transfer circuit each including an SCR switch circuit.
 3. The circuit of claim 2 wherein the energy transfer circuit comprises a bypass circuit, the bypass circuit being coupled in parallel with the third leg of the energy transfer circuit, and wherein the bypass circuit is configured to provide a conductive path that bypasses the third leg of the energy transfer circuit during the pacing mode and is configured to open circuit the conductive path during the defibrillation mode.
 4. The circuit of claim 3 wherein the defibrillation pulse is selectably provided as a biphasic pulse and the pacing pulse is provided as a monophasic pulse.
 5. The circuit of claim 3 wherein, during the pacing mode, the control circuit is configured to determine the predetermined level in charging the energy storage capacitor to achieve a predetermined current level for a subsequently provided pacing pulse.
 6. The circuit of claim 2 wherein the H-bride circuit comprises a current source circuit, the current source circuit being coupled in parallel with the third leg of the energy transfer circuit, and wherein the current source circuit is configured to provide a configurable current to the first output lead during the pacing mode and is configured to provide essentially no current to the first output lead during the defibrillation mode.
 7. The circuit of claim 6 wherein, during the pacing mode, the control circuit is configured to cause the current source circuit to provide the configurable current with a predetermined current level.
 8. The circuit of claim 6 wherein the current source circuit comprises an IGBT and a resistor.
 9. The circuit of claim 1 wherein the energy transfer circuit comprises four legs, a first leg and a second leg of the energy transfer circuit each including an IGBT switch circuit, and a third leg and a fourth leg of the energy transfer circuit each including an SCR switch circuit.
 10. The circuit of claim 9 wherein the H-bride circuit comprises a bypass circuit, the bypass circuit being coupled in parallel with the third leg of the energy transfer circuit, and wherein the bypass circuit is configured to provide a conductive path that bypasses the third leg of the energy transfer circuit during the pacing mode and is configured to open circuit the conductive path during the defibrillation mode.
 11. The circuit of claim 10 wherein the defibrillation pulse is selectably provided as a biphasic pulse having a first phase with a first polarity and a second phase of a second polarity, and wherein the pacing pulse is provided as a monophasic pulse with the second polarity.
 12. The circuit of claim 11 wherein, during the pacing mode, the control circuit is configured to determine the predetermined level in charging the energy storage capacitor to achieve a predetermined current level for a subsequently provided pacing pulse.
 13. The circuit of claim 9 wherein the H-bride circuit comprises a current source circuit, the current source circuit being coupled in parallel with the third leg of the energy transfer circuit, and wherein the current source circuit is configured to provide a configurable current to the first output lead during the pacing mode and is configured to provide essentially no current to the first output lead during the defibrillation mode.
 14. The circuit of claim 13 wherein, during the pacing mode, the control circuit is configured to cause the current source circuit to provide the configurable current with a predetermined current level.
 15. The circuit of claim 13 wherein the current source circuit comprises an IGBT and a resistor.
 16. The circuit of claim 1 wherein the energy transfer circuit comprises four legs, each of the four legs of the energy transfer circuit including an IGBT switch circuit.
 17. The circuit of claim 16 wherein the defibrillation and pacing pulses are selectably provided as a biphasic pulse or monophasic pulse.
 18. The circuit of claim 17 wherein the defibrillation pulse is a biphasic pulse having a first phase with a first polarity and a second phase of a second polarity, and wherein the pacing pulse is a monophasic pulse with the second polarity.
 19. The circuit of claim 17 wherein the defibrillation pulse is a biphasic pulse having a first phase with a first polarity and a second phase of a second polarity and the pacing pulse is a biphasic pulse having a first phase of the second polarity and a second phase of the first polarity.
 20. The circuit of claim 16 wherein, during the pacing mode, the control circuit is configured to determine the predetermined level in charging the energy storage capacitor to achieve a predetermined current level for a subsequently provided pacing pulse.
 21. The circuit of claim 16 wherein the H-bride circuit comprises a current sense circuit coupled to the energy storage capacitor and the control circuit, and wherein the current sense circuit is configured to detect a current level of current provided by the energy storage capacitor when the circuit is providing a pacing pulse.
 22. The circuit of claim 21 wherein, during the pacing mode, the control circuit is configured to cause the third and fourth legs of the energy transfer circuit to conduct a predetermined level of current when the circuit is providing a pacing pulse.
 23. The circuit of claim 13 wherein the current sense circuit comprises an amplifier, a transformer and a resistor.
 24. A method of externally providing a defibrillation pulse or a pacing pulse to a patient from a single unit, the method comprising: charging an energy storage capacitor; during a defibrillation mode, transferring energy from the energy storage capacitor to the patient in a defibrillation pulse; and during a pacing mode, transferring energy from the energy storage capacitor to the patient in a pacing pulse.
 25. The method of claim 24 wherein an energy transfer circuit is used to transfer energy from the energy storage capacitor to the patient in both the defibrillation and pacing modes.
 26. The method of claim 24 wherein the energy storage capacitor is charged to a predetermined level so that the pacing pulse has a current of a predetermined level.
 27. The method of claim 24 wherein the pacing pulse is a biphasic pulse.
 28. An apparatus for externally providing to a patient a defibrillation pulse during a defibrillation mode and a pacing pulse during a pacing mode, the apparatus comprising: an energy storage capacitor; charging means for charging the energy storage capacitor; switch means, coupled to the energy storage capacitor, for selectively transferring energy from the energy storage capacitor to the patient; and control means for causing the switch means to transfer energy from the energy storage capacitor to the patient in a defibrillation pulse during the defibrillation mode, and for transferring energy from the energy storage capacitor to the patient in a pacing pulse during the pacing mode.
 29. The apparatus of claim 28 wherein the switch means comprises an energy transfer circuit, wherein the energy transfer circuit is selectively configurable to transfer energy from the energy storage capacitor to the patient in both the defibrillation and pacing modes.
 30. The apparatus of claim 28 wherein the control means is configured to cause the charging means to charge the energy storage capacitor to a predetermined level wherein the pacing pulse has a current of a predetermined level.
 31. The apparatus of claim 28 wherein the pacing pulse is a biphasic pulse. 